This project tested for the presence of several viral antibodies in adult and juvenile Macacas on Cayo Santiago, Puerto Rico. This has been a closed colony of Rhesus monkeys since 1938. A serological screen carried out in the early 1950's indicated the presence of antibody to SV 40 (46%), herpes B (27%) and measles virus (80%) of the animals in the colony. The objective was to determine, after 40 years as a closed colony, if herd immunity to the three previously studied antigens has been lost, and thus provide an animal population useful for the testing of related strains of viruses. With the tremendous interest and need for animal models of AIDS and other human diseases, the following viral antigens were added to the testing protocol: human T-lymphotropic virus I (HTLV-I), human immunodeficiency virus (HIV), rhesus CMV, simian retrovirus D (SRV-I) and simian T-lymphotropic virus type I (STLV-III). In addition, the scope of testing was increased to include animals in the NINDS breeding colony in Puerto Rico. The percent of animals positive for SV40, rubeola, rhesus CMV, HTLVI and herpes-B were: 0.5%, 10.3%, 97.1%, 72.5% and 81.4%, respectively. All animals were serologically negative for SRV-I, STLV-III and HTLV-III (HIV) and are appropriate for research studies in AIDS. A manuscript reporting the results has been accepted for publication.